When the news suggests prescription drug abuse is going on, what factors did they check that suggest prescription drug abuse? Like when they say pain pills are overused, how do they know it's not just for actual pain? What are these "signs" of prescription pain killer abuse?

I know that an overdose means you have taken too much drugs at once, but what does that mean for your body? What is the science behind a drug overdose (OD)? How do people die from overdosing? Can you survive an OD?